The present invention relates to a novel retardation film. More specifically, it relates to a retardation film with novel optical properties that is useful for an optical element such as a liquid crystal display device or anti-glare film, to a laminated retardation film and retardation film-integrated polarizing film employing it, and to optical apparatuses such as liquid crystal display devices that employ the films.
Retardation films are used in STN (Super Twisted Nematic) systems of liquid crystal display devices, with the aim of solving issues such as color compensation or viewing angle enlargement. The materials commonly used for color compensating retardation films are polycarbonates, polyvinyl alcohols, polysulfones, polyether sulfones, amorphous polyolefins and the like, while the materials used for viewing angle enlarging retardation films are those mentioned above as well as polymer liquid crystals, discotic liquid crystals and the like.
A quarter-wave film, which is one type of retardation film, can convert circularly polarized light to linearly polarized light, or linearly polarized light to circularly polarized light. This has been utilized in combination with liquid crystal display devices, particularly reflective liquid crystal display devices having a single polarizing film where the rear electrode, as viewed by an observer, is the reflecting electrode, with anti-reflection films comprising a combination of a polarizing film and a quarter-wave film, or with reflective polarizing films composed of cholesteric liquid crystals or the like that reflect only circularly polarized light only in either the clockwise direction or counter-clockwise direction.
The retardation films used in the aforementioned single polarizing film-type reflective liquid crystal display devices and reflective polarizing films must have a function of converting linearly polarized light to circularly polarized light and circularly polarized light to linearly polarized light, in the visible light region with a measuring wavelength of 400-700 nm, and preferably 400-780 nm. When this is accomplished with a single retardation film, the retardation film ideally has a retardation of xcex/4 (nm) (100-175 nm, preferably 195 nm) at a measuring wavelength xcex of 400-700 nm, and preferably 400-780 nm.
In order to achieve a smaller retardation with a shorter measuring wavelength as with an ideal quarter-wave film, Japanese Unexamined Patent Publication (Kokai) HEI No. 10-68816 has disclosed a technique of using a quarter-wave film and a half-wave film attached together at an appropriate angle. According to this method, when linear polarized light is incident to the film at an appropriate angle, satisfactory circularly polarized light is obtained in approximately the wavelength range of the visible light region. However, the method of Japanese Unexamined Patent Publication HEI No. 10-68816 requires the quarter-wave film and half-wave film to be attached at an angle which is not perpendicular or parallel to the slow axis in the in-plane direction of each film. Because retardation films made of polymer materials are usually fabricated in a roll-to-roll manner in a stretching process, the slow axis in the in-plane direction of the film lies parallel or perpendicular to the direction in which the film runs, although this depends on the stretching method and the refractive index anisotropy of the film material. Consequently, attachment in such a manner that the retardation axes of the in-plane direction of each film are not at a perpendicular or parallel angle is not preferred from the standpoint of productivity because, when used in a liquid crystal display device, for example, the attachment step is carried out after cutting to the desired size, and there is a reduction in cutting yield while it is essentially impossible to attach the two films continuously in a roll-to-roll manner.
Japanese Patent No. 2609139 discloses a laminated retardation film characterized in that two, three or more different birefringent films made of transparent stretched plastic films are laminated with a combination of different retardation wavelength dependencies due to birefringence, and in the case of a laminate of two different birefringent films, the directions of the maximum in-plane refractive index are in a non-perpendicular relationship by a combination with different signs for the oriented birefringence, or a combination with the same signs for the oriented birefringence is used. The method of Japanese Patent No. 2609139 also allows a certain degree of retardation control, but it requires the use of a plurality of positive or negative films.
In Japanese Examined Patent Publication (Kokai) HEI No. 6-230368 there is also disclosed a retardation film comprising a laminate of stretched films of two or more different polymers, wherein the birefringence is zero at at least one wavelength of visible light. However, since the method of Japanese Unexamined Patent Publication HEI No. 6-230368 also attaches two or more polymer films, extra effort is required for the attachment step in which an optically satisfactory combination film is obtained, just as in the case of Japanese Patent No. 2609139, and therefore the cost is increased for the production of two or more films, while the small thickness of the film is an additional disadvantage.
It is a principal purpose of the present invention to provide a retardation film that compensates for the optical properties of a liquid crystal cell in a liquid crystal display device, to give enhanced image quality.
It is another object of the invention to provide a single retardation film with novel optical properties of its own.
It is yet another object of the invention to provide a novel laminated retardation film or retardation film-integrated polarizing film with improved optical qualities over other retardation films or polarizing films by combination with the aforementioned retardation film.
It is yet another object of the invention to provide a retardation film that is useful for optical apparatuses such as liquid crystal display devices.
The present inventors have studied a wide variety of materials with excellent optical properties, but for the optical uses of retardation films, attention was focused on polymer materials as transparent materials with low light absorption at the measuring wavelength, materials with a glass transition temperature of 100xc2x0 C. or higher, preferably 120xc2x0 C. or higher and especially 150xc2x0 C. or higher, and materials that exhibit favorable molding properties. Polymer materials may be crystalline, amorphous, or liquid crystalline, but amorphous polymers usually allow solvent casting process and are therefore preferred for purposes in which retardation irregularities and the like must be minimized, such as with retardation films. From this standpoint, polycarbonates, polyesters, polyallylates, polyolefins and the like are best as polymer materials, but it is believed that polycarbonates are particularly advantageous from the viewpoint of productivity and increasing freedom of molecular design for copolymerization and the like.
The present inventors also researched the optical properties, and found that a polymer film with excellent properties as a retardation film can be obtained by stretching a polymer film composed of a polymer blend comprising a polymer with positive refractive index anisotropy and a polymer with negative refractive index anisotropy, a copolymer made from a monomer component of a polymer with positive refractive index anisotropy and a monomer component of a polymer with negative refractive index anisotropy, or a combination thereof. The polymers with positive and negative refractive index anisotropy referred to here are defined as follows: a polymer with positive refractive index anisotropy is one wherein the direction of the maximum refractive index in the in-plane direction of the film, i.e. the slow axis, matches the stretching direction when the polymer film is uniaxially stretched, and a polymer with negative refractive index anisotropy is one wherein the slow axis is roughly perpendicular to the stretching direction. Some materials, like polystyrene, have positive refractive index anisotropy or negative refractive index anisotropy depending on the conditions of uniaxial stretching, but here it is defined as the refractive index anisotropy exhibited upon uniaxial stretching from 10xc2x0 C. below the glass transition temperature to 20xc2x0 C. above the glass transition temperature, as the usual stretching temperature conditions for fabrication of a commercially available retardation film. These measurements are made by polarized light analysis at a wavelength of 550 nm.
Retardation films characterized by having a range in which the retardation value is positive and a range in which it is negative in a measuring wavelength range of from 400 to 800 nm with a single retardation film are unknown to the prior art. The present inventors have conducted diligent research on materials that give such retardation films, and have completed the present invention upon the discovery that polymers such as certain polycarbonates and blends of polyphenylene oxide and polystyrene are effective to this purpose, and that such retardation films can be fabricated by appropriate selection of the polymers.
It was further found that such retardation films can be used as laminates with other retardation films to control the retardation wavelength dispersion of the other retardation films and thus contribute to enhanced image quality for liquid crystal display devices.
In other words, the present invention is accomplished by a retardation film consisting of a single polymer film, which has a wavelength range in which the retardation value is positive and a wavelength range in which it is negative in a wavelength range of 400-800 nm, which satisfies the following inequality (1) and/or (2), and which has a water absorption of no greater than 1% by mass.
|R(400)|xe2x89xa710 nmxe2x80x83xe2x80x83(1)
|R(700)|xe2x89xa710 nmxe2x80x83xe2x80x83(2)
where |R(400)| and |R(700)| represent the retardation values at wavelengths of 400 nm and 700 nm, respectively.
The retardation film of the invention is believed to be based on the following principle. That is, presumably, when the retardation value of the component with positive refractive index anisotropy and the retardation value of the component with negative refractive index anisotropy completely cancel each other out, the retardation value is exactly zero. However, because the polymer has wavelength dispersion in its birefringence, appropriate adjustment of the amount of the component with the positive refractive index anisotropy and the amount of the component with the negative refractive index anisotropy produces a phenomenon such that the retardation value is exactly zero at a wavelength in the measuring wavelength range of 400-800 nm, but the signs of the retardation values in the adjacent ranges are switched, thus giving a positive range and negative range for the retardation value in the measuring wavelength range of 400-800 nm.